The present invention relates to an ultrasonic microscope apparatus for observing the internal structure of an object being measured by ultrasonic waves.
Integrated circuits (ICs), for example, are manufactured through many process steps. In the course of manufacturing ICs, IC chips sometimes sustain defects. If a defect is located in an important region of the IC chip, the electrical performance of the IC is seriously degraded. Most of the defects appear in the boundary layers in the area of the chip surface. Such defects cannot be inspected by an optical microscope. To cope with this problem, an ultrasonic microscope apparatus has been proposed as an effective means for inspecting defects and determining the resiliency of the material in the surface region of the object under measurement.
To form an image with an ultrasonic microscope apparatus, ultrasonic waves are irradiated from a sound source and converged, by an acoustic lens, onto a microspot. The microspot is used to achieve a two-dimensional scanning of the surface of the object. The ultrasonic waves reflected and scattered by the object under measurement are collected and converted into electrical signals. In synchronism with the scanning of the ultrasonic microspot, the electrical signals are two-dimensionally visualized on a CRT, for example, in the form of an ultrasonic image.
FIG. 1 illustrates, in block form, a conventional ultrasonic microscope apparatus. High frequency pulses generated by a pulse generator 1 are applied, through a buffer amplifier 2 and a circulator 3, to a piezoelectric transducer 5 provided near the rear end of an acoustic lens 4. The piezoelectric transducer 5 made of zinc oxide (ZnO), for example, generates an ultrasonic wave of a frequency corresponding to that of the high frequency pulse voltage and an amplitude proportional to the pulse voltage amplitude. The ultrasonic wave is converged by the acoustic lens 4, which may be, for example, a saffire rod of a plano-convex shape. As shown, the acoustic lens 4 is so arranged that its plano side is positioned near the piezoelectric transducer 5, and its convex side is polished and shaped in such a manner as to have a spherical surface. The acoustic lens 4 converges ultrasonic waves onto a microspot on an object under measurement 6, such as an IC chip. The space between the spherical surface of the acoustic lens 4 and the object 6 is filled with a medium 7 such as liquid or gas. When gas is used as the medium 7; as its pressure is increased, the resolution of the ultrasonic image is improved. The ultrasonic wave is reflected or scattered from the object depending on the acoustical characteristics of the surface or surface region of the object. Reflected or scattered ultrasonic waves are collected by the acoustic lens 4. The collected ultrasonic wave energy is converted, by the piezoelectric transducer 5, into an electrical signal; and is applied to a gate 8, through the circulator 3. The gate 8 selects the necessary information signals and applies them to a mixing circuit 11, through an attenuator 9 and a high frequency amplifier 10. In the mixing circuit 11, the selected signal is mixed with a signal output from a local oscillator 12, and is converted into an intermediate frequency signal. The intermediate frequency signal is amplified by an intermediate frequency amplifier 13. The output signal of the intermediate frequency amplifier 13 is detected by a detector 14, and is then applied to a scanning converter 18, via a blanking circuit 15, a peak detector 16 and a limiter circuit 17.
A control unit 19 drives, through a scanning drive circuit 20, a stage 21 with the object 6 placed thereon, allowing the object under measurement 6 to be scanned by the ultrasonic spot. For Y-direction scanning of the object 6, the stage 21 is driven at a low rate of speed, using a lead screw (not shown). For X-direction scanning, the acoustic lens 4 is driven at a high speed using a voice coil actuator. The Y and X-direction scanning movements are concurrently performed to produce two dimensional scanning. In synchronism with this scanning, the control unit 19 applies a signal to the gate 8, the blanking circuit 15 and the scanning converter 18, to produce brightness signals at individual positions in the vicinity of the surface of the object 6. The brightness signals are applied to a monitor 22; which, in turn, visualizes them as an image of the object 6, in synchronization with the two-dimensional scanning operation.
The ultrasonic microscope apparatus thus constructed, when employed in inspecting defects in and on integrated circuits, for example, can inspect defective portions of the integrated circuits, which portions cannot be found by observation of only the surfaces of the IC chips. Thus, the ultrasonic microscope apparatus can accurately inspect defective portions in the semiconductor structure. The ultrasonic microscope apparatus is thus very useful when applied to the testing of high density integrated circuits of the multilayered structure type, which circuits have gradually come into use in recent years.
The aluminum interconnection wires of IC chips may have protruding portions. Thermal stresses generated under operating conditions cause those protruding portions to contact with other wires or parts. To determine the cause of the defect, it is necessary to recreate the same conditions which caused the defect. Toward this end, the electrical operating conditions and ambient conditions, e.g., temperature and humidity, are changed every measurement to observe the internal structure of the IC chip. The ultrasonic microscope apparatus of FIG. 1 requires a manual operation for setting and changing the operating and ambient conditions.
Further, the results of measurement must be photographed, for the purpose of recording them. The actual photograph is, of course, troublesome for an operator, accompanied by complicated and highly skilled operations. Careful comparison of the photographs taken is essential for proper defect inspection. This comparative, visual check is based on the judgment faculties of the operator. Therefore, to obtain an accurate and reliable check, the inspectors must be highly skilled. Otherwise, the results of the inspection may frequently be unreliable; as well as impracticable, in terms of the time consumed.